Alumina values can be recovered from aluminiferous ores in a variety of ways. One widely utilized process is the well known Bayer process which involves digestion of the aluminiferous ore in a caustic solution at elevated temperatures. The digestion results in a caustic aluminate solution and a caustic insoluble residue commonly called "red mud." In the usual Bayer process, the caustic aluminate solution is separated from the red mud and subjected to precipitation to enable recovery of the alumina values. The red mud, which also contains occluded or trapped caustic soluble alumina values is generally washed to remove most of these values from the mud in order to improve the yield of the Bayer process.
However, in many instances, the red mud also contains considerable quantities of alumina which, under conditions of the Bayer process, is unavailable to caustic extraction processes and constitutes unrecoverable values. This is particularly true when the ore which is subjected to the Bayer processing has an appreciable quantity of silica because the silica and the alumina react during the Bayer process to form an insoluble desilication product, thereby causing a loss in alumina and caustic values.
It has been heretofore proposed in the art to subject low iron-containing aluminiferous ores, including the aforementioned "red mud" to the so-called lime-soda-sinter process. In this process, an alkaline earth compound such as lime, and an alkali metal compound, such as soda ash, is admixed with the red mud and sintered. The function of the alkaline earth compound is to react with the silica and to form an insoluble compound of calcium and silica. The function of the alkali metal compound is to react with the alumina and to form a soluble alkali metal aluminate. After the sintering has been complete, the sinter is then leached for the purpose of recovering therefrom soluble alumina and caustic values.
Although the lime-soda-sinter process has been known in the art, there have been many problems associated therewith in an attempt to arrive at an economic balance between the cost involved in recovering the added values of alumina and alkali, and the cost of the treatment itself. There have also been technical problems in an attempt to recover the maximum amount of alumina and caustic values. Thus, for example, the sintering operation must be carried out in such a manner as to cause fritting or sintering of the particles without substantial fusion or melting of large portions of the mixture. Melting of substantial portions of the mass would interfere with the recovery of the values during subsequent leaching operations, thereby seriously affecting the overall efficiency.
The prior art was aware of various techniques for treating aluminiferous ores having a high iron content. The art was aware that the pressure of high iron oxides content in the ore involved an irretrievable loss of limestone or soda ash reagents and usually resorted to alternate treatments in order to reduce the iron oxide content of the ore. Thus, for example, it is known in the art to incorporate into the lime-soda-sinter treatment process a carbonaceous material which was stated to react with the iron oxides to form magnetic iron which could then be separated by other techniques. Other proposed methods suggested in the art were to use an acid extraction step so as to produce crude alumina contaminated with iron, but which was relatively free of silica and then to subject this product to the lime-soda-sinter process. Quite obviously, the combination of an acid and a caustic process can lead to additional operating expenses. However, the prior art did not devote too much attention to processing a Bayer red mud having a high iron content. Quite obviously, an acid process would destroy the soda values and the heretofore mentioned magnetic separation techniques might have been thought to be too costly.
As is known, a Bayer red mud contains a substantial amount of a zeolitic desilication product admixed with iron, titanium and/or other materials not normally soluble in caustic. The zeolitic material or desilication product generally has a formula expressed as mole ratio of oxide: EQU 4 Na.sub.2 O : 3 Al.sub.2 O.sub.3 : 3 SiO.sub.2 : Y
wherein Y is SO.sub.3, Cl,OH, and CO.sub.2 depending on the impurities in the Bayer liquor from which the zeolite was precipitated. It appears that in the process of this invention, a Bayer red mud acts differently than aluminiferous ores generally.